Improving driving safety is one major objective of forming Vehicular Ad hoc NETworks (VANETs). Existing VANETs usually assume drivers detect and report safety-related road conditions. However, drivers may not be willing to perform these duties; even they are, these duties may distract them from driving and thus make driving unsafe. To address the problem, this paper proposes an automatic detection system. By taking advantage of the communication capability of roadside sensors, the proposed system can automatically detect and locate problematic road conditions without any human intervention under varying traffic densities. Extensive simulations have been conducted to verify the efficiency of the proposed system.